Take at look at what Sun did with Java. They opened everything. Compiler, JDK, development environment... sounds good, doesn't it?
Well... I think they are just lying. They opened compiler v7. They opened JDK v6 and v7. But v5 is the current version and v4 is the most widespread incarnation. So they didn't risk anything, they got free propaganda but only opened future versions, not current versions.
Microsoft is engaged in .NET and Java could be at the beginning of its end. So they open their platform and try to add momentum and propaganda from the Open Source Community.
It's not fair. It's a trap.